Interplay of Voltage Control of Magnetic Anisotropy, Spin-Transfer Torque, and Heat in the Spin-Orbit-Torque Switching of Three-Terminal Magnetic Tunnel Junctions

Abstract

We use three-terminal magnetic tunnel junctions (MTJs) designed for field-free switching by spin-orbit torques (SOTs) to systematically study the impact of dual voltage pulses on the switching performances. We show that the concurrent action of an SOT pulse and an MTJ bias pulse allows for reducing the critical switching energy below the level typical of spin transfer torque while preserving the ability to switch the MTJ on the sub-ns time scale. By performing dc and real-time electrical measurements, we discriminate and quantify three effects arising from the MTJ bias: the voltagecontrolled change of the perpendicular magnetic anisotropy, current-induced heating, and the spin transfer torque. The experimental results are supported by micromagnetic modeling. We observe that, depending on the pulse duration and the MTJ diameter, different effects take a lead in assisting the SOTs in the magnetization reversal process. Finally, we present a compact model that allows for evaluating the impact of each effect due to the MTJ bias on the critical switching parameters. Our results provide input to optimize the switching of three-terminal devices as a function of time, size, and material parameters.

Magnetisation switching dynamics induced by combination of spin transfer torque and spin orbit torque

“…neglecting other effects such as Joule heating, it can be clearly seen that low write energy can be achieved by optimising and . The obtained write energy is of the order of hundreds of fJ, in agreement with the results reported in literature for this kind of devices and also with technological requirements 9 , 10 , 34 , 35 , 42 , 45 , 46 , 63 , 64 . Increasing results in high energy consumption, whereas the combination of SOT and STT currents assists in the reduction of write energy.…”

“…Our results suggest that for small MTJ diameters even a weak is able to make the reversal coherent. To induce non-uniformities such as nucleation processes in this combined STT-SOT system larger diameters 39 , 45 , 46 and/or strong current densities would be required. As such, our assumption of spatially uniform current densities should not impact the results presented here.…”

“…Such a MTJ dimension is characterised by non-coherent magnetisation dynamics, something undesirable in such applications, and is not feasible for high storage applications. Grimaldi et al 45 and Krizakova et al 46 demonstrate very promising SOT-induced switching assisted by STT and VCMA in three terminal CoFeB-based MTJs. However, to achieve a deterministic reversal with 100% switching probability an external in-plane magnetic field is applied in the case of SOT-induced dynamics.…”

“…When STT assists SOT-driven dynamics, STT can provide the necessary bias to break the symmetry and provide deterministic switching of the magnetisation. This removes the necessity of an external field or to resort to engineered magnetic structures 8 , 11 , 28 , 44 – 46 reducing the required current density. Most of the studies on SOT-MRAM assisted by STT focused on three-terminal devices 11 , 37 , 45 , 46 , even though there are reports of two-terminal devices 38 , 39 .…”

“…This removes the necessity of an external field or to resort to engineered magnetic structures 8 , 11 , 28 , 44 – 46 reducing the required current density. Most of the studies on SOT-MRAM assisted by STT focused on three-terminal devices 11 , 37 , 45 , 46 , even though there are reports of two-terminal devices 38 , 39 . SOT-assisted STT-MRAMs are investigated in Ref.…”